The present invention relates to a sheet feeder provided in an image forming device such as a copy machine, laser printer, and facsimile machine, and more particularly, to a sheet feeder for feeding stacked sheets of a recording medium one sheet at a time.
A conventional sheet feeding device provided in image forming devices such as copy machines, laser printers, and facsimile devices is shown in FIGS. 10 and 11. This sheet feeding device is provided with a feed roller 101 for conveying a paper 102 and a separating pad 103 opposing the feed roller 101 such that the paper 102 is interposed between the feed roller 101 and separating pad 103. This sheet feeding device is well known in the art for separating and conveying each of a stack of paper 102 one sheet at a time. The separating pad 103 is embedded into a depression formed in a sheet receiving portion 105 of a separating pad holder 104. The separating pad holder 104 is urged toward the feed roller 101 by a spring 107 interposed between a fixed stop member 106 and the bottom surface of the sheet receiving portion 105. The feed roller 101 is formed of an elastic material in an approximate D-shape and includes a circumferential surface 108 for contacting and conveying the paper 102 and a cut-out surface 109 formed by cutting away a portion of the circumferential surface 108. Collars 110 are rotatably mounted on both ends of the feed roller 101. Both of the collars 110 are formed with a radius slightly smaller than the radius of the circumferential surface 108. Hence, when the circumferential surface 108 is conveying the paper 102, the collars 110 do not contact the paper 102. However, when the circumferential surface 108 has completed conveying the paper 102 and the cut-out surface 109 opposes the paper 102, the peripheral surface of the two collars 110 contact the paper 102 and oppose the urging force of the separating pad 103, thereby forming a space between the cut-out surface 109 and the paper 102 to prevent the cut-out surface 109 from contacting the paper 102.
Guide rollers 112 and 113 are positioned downstream of the feed roller 101 and convey the paper 102 after the circumferential surface 108 has completed conveying the paper 102. The cut-out surface 109 is separated a prescribed distance from the paper 102 by the two collars 110, thereby stopping driving of the paper 102. In addition, the collars 110 contact and rotate on the top surface of the paper 102, which is conveyed by the guide rollers 112 and 113, thereby allowing only one paper 102 to be conveyed at a time.
However, when conveying the paper 102, the circumferential surface 108 and the collars 110 contact the paper 102 and separating pad 103 at different positions, or heights, because the radius of the two collars 110 is smaller than the radius of the circumferential surface 108. This difference in radii creates a bump when the paper 102 and separating pad 103 change from contacting the collars 110 to contacting the circumferential surface 108 and vice versa, causing the separating pad 103 to move upward and downward.
When the separating pad 103 moves upward and downward, as described above, there is a tendency for a plurality of paper 102 to be fed simultaneously, causing doubles or partly overlapped sheets to be fed. In other words, when the paper 102 is first fed under the feed roller 101, the leading edge of the paper 102 contacts an entry contact portion 115 of the circumferential surface 108, which is the transition point from the collars 110 contacting the separating pad 103 to the circumferential surface 108 contacting the separating pad 103. Therefore, the separating pad 103 is moved downward. In this instant, the pressure on the paper 102 changes greatly, inviting a plurality of paper 102 to enter simultaneously between the circumferential surface 108 and separating pad 103, inviting the feeding of doubles or multiple sheets. Also, when the circumferential surface 108 has completed conveying the paper 102, an exit contact portion 114 of the circumferential surface 108 is the last part of the feed roller 101 to contact the paper 102 before the roller 110 come in contact with the paper 102. In the instant that the paper 102 separates from the circumferential surface 108, the separating pad 103 moves upward, causing a great change in pressure on the paper 102, inviting the next paper 102 or plurality of next paper 102 to be conveyed along with the paper 102 being conveyed between the guide rollers 112 and 113, inviting the feeding of partly overlapped sheets. That is, immediately before stopping rotation of the sheet feed roller 101, the front part of the sheet reaches the guide rollers 112, 113, and the sheet is fed by the guide rollers 112,113. Further, the pressure from the separation pad 103 to the sheet is temporarily lowered when the paper 102 and separating pad 103 change from contacting the circumferential surface 108 of the feed roller 101 to contacting the collars 110. In this case, the subsequent sheet may also be fed in accordance with the advancing movement of the precedent sheet due to the frictional force between the precedent sheet and a subsequent sheet.
Further, the non circular cross-sectional shape of the feed roller 101, i.e., D-shape in cross-section incurs high material cost, and an entire feed mechanism becomes complicated and causes high production cost.